Titanium is a light metal having a high mechanical strength to weight ratio and exhibiting superior corrosion resistance. Titanium is widely used in various fields including the airplane, medical and automobile industry. The amount of titanium being used has been steadily increasing at a yearly rate of 5%. Titanium is commonly found in the earth's crust, being the 10th most abundant element. Although the demand for titanium has been steadily increasing, its production has not increased sufficiently to satisfy the demand. The Kroll method, the currently used industrial titanium refining method, suffers from low production efficiency and the production of titanium metal in the form of a porous sponge.
In the Kroll method, titanium ore, the main component of which is titanium dioxide (TiO2), is reacted with chlorine gas and coke (C) to provide titanium tetrachloride (TiCl4) which is subsequently purified. The purified titanium tetrachloride is reduced using magnesium to provide titanium metal. The process can be summarized as follows:TiCl4+2Mg→Ti+2MgCl2 
The various process steps for the production of titanium metal following the Kroll method are illustrated in FIG. 1. Titanium ore, comprising titanium dioxide TiO2, is reacted with chlorine gas and coke to provide titanium tetrachloride (step 2). The titanium tetrachloride product is refined and purified (step 3). Titanium tetrachloride, in a liquid state at room temperature, is subsequently reduced in a reaction vessel comprising molten magnesium (step 4). The reduced titanium tetrachloride generates sponge titanium. The magnesium chloride by-product, as well as any unreacted magnesium, is removed from the sponge titanium product by transferring the reaction mixture to another vessel which is heated at a temperature above the boiling point of magnesium chloride. The magnesium chloride vapor is drawn-off by the application of a vacuum (step 5). The resulting titanium sponge is removed from the vessel and cut into large chunks or ground into a powder (step 6). Alloying elements such as aluminum or vanadium can optionally be added (step 7). The product is compacted to form an electrode (step 8) which is subsequently melted (step 9). The molten material is either atomized to produce a fine spherical powder (step 10) which is subsequently sintered (step 11), or cast (step 12) to provide a material which is worked to provide a wrought material (step 13). The Kroll method is a complicated batch process which requires many process steps and which requires extensive amounts of production time. A batch comprising 9 tons of titanium will typically require 7-9 production days.
The Kroll method typically comprises the use of a stainless steel vessel filled with a magnesium melt maintained at a temperature of 800° C. Titanium tetrachloride, in the liquid phase, is introduced into the vessel and reacts with the magnesium melt to generate titanium metal. The freshly generated titanium metal sinks in the magnesium melt, while the concomitantly produced magnesium chloride remains in the melt in the liquid phase. Thus, the resultant reaction mixture comprises titanium metal and a liquid phase composed of magnesium and magnesium chloride. Upon completion of the reaction, the reaction mixture is subjected to an in vacuo, high temperature separation process to provide a sponge cake of porous titanium. Since titanium is an active metal that easily reacts with oxygen or nitrogen, it is important that exposure to the air be kept at a minimum. The reaction vessel is thus sealed following the introduction of the titanium tetrachloride feed material.
A large amount of porous titanium sponge cake is obtained following the magnesium mediated reduction reaction and subsequent separation process. However, as the amounts of oxygen and other impurities vary between the core and the periphery of the cake, the material is not suitable for use as an electrode for vacuum arc re-melting (VAR). The sponge cake must be crushed such that the core and the periphery become fractionated.
The production of titanium metal by means of the Kroll method requires long periods of time, typically 10 days, as the reduction and separation process are carried-out batch wise and the process requires a subsequent crushing step. These process limitations adversely affect the production cost.
In an embodiment of the Kroll method, a pair of stainless steel vessels is used such that the porous titanium sponge cake, obtained following the reduction process, can also be subjected to the separation process. While this embodiment provides for a titanium product of enhanced quality, the production efficiency does not meet the increasing market demand.
Several alternative methods have been developed for the production of titanium metal.
U.S. Pat. No. 6,712,952 issued to Fray et al. on Mar. 30, 2004 discloses a method for obtaining titanium metal through the electrolysis of titanium oxide in a molten salt bath. However, product quality and process efficiency are some of the drawbacks rendering the method unsuitable for large scale production.
JP-A-7-252550 discloses a method for producing titanium metal by reducing titanium tetrachloride using an alkali metal or alkali earth metal. A mixture of titanium metal and alkali metal or alkali earth metal halide is obtained. The mixture is fed into a crucible having open upper and lower ends and heated using a plasma torch such that the titanium metal is melted and the halide by-product is vaporized. The molten titanium metal is drawn downward and isolated as an ingot. The separation of the by-product and the formation of the ingot are thus performed in a single process step.
JP-A-7-278691 discloses a further method for producing titanium metal by reducing titanium tetrachloride using magnesium metal. Titanium metal is introduced into a crucible having open upper and lower ends and molten (e.g. RF heating or RF plasma heating). Titanium tetrachloride and magnesium metal are subsequently fed into the molten pool of titanium. The magnesium chloride by-product is vaporized and removed. The method provides for the continuous production of titanium ingots.
JP-A-58-110626 discloses a further method for producing titanium metal in which a gas mixture (e.g. Ar, He and/or H2) is passed between arc-heater electrodes to generate an arc jet which is subsequently discharged into a reactor vessel. Molten Mg or Na and titanium tetrachloride are fed into the vessel. The titanium tetrachloride feed is reduced by the sodium or magnesium and titanium metal is deposited on the wall of the vessel.
U.S. Pat. No. 4,080,194 issued to Fey on Mar. 21, 1978 discloses a method for obtaining titanium metal by injecting titanium tetrachloride and magnesium or sodium into a reaction chamber heated by an arc jet. The reduced titanium is collected in a mold as a titanium ingot.
The present disclosure refers to a number of documents, the content of which is herein incorporated by reference in their entirety.